Sonic the Hedgehog 2 CD

From Sonic Retro

Sonic 2 CD was either a development name for Sonic the Hedgehog CD or an enhanced port of Sonic the Hedgehog 2 (16-bit). Many video game magazines referred to the upcoming Sonic game for Sega CD as an enhanced Sonic 2 port. When it was thought to be a port of Sonic 2 it was said to have CD quality music and additional levels that the Mega Drive counterpart did not.

There is also evidence of the port never existing as some people involved in Sonic 2's development have been contacted by Sonic fans and could not remember a CD port.